1. Field of the Invention
The present invention relates to an exposure apparatus which projects the pattern of an original onto a substrate by a projection optical system within a chamber to expose the substrate, and a method of manufacturing a device using the exposure apparatus.
2. Description of the Related Art
In recent years, an exposure apparatus for manufacturing devices such as a semiconductor device has been required to further improve resolution, overlay accuracy, and throughput. In the current mass production line of semiconductor devices, a high-resolution exposure apparatus is commonly used to pattern a critical layer, whereas a high-throughput exposure apparatus which has a relatively low resolution but a wide angle of view is commonly used to pattern a non-critical layer.
In this manner, an improvement in overlay accuracy is of prime importance in processes which use a plurality of exposure apparatuses having different specifications. To improve the overlay accuracy, it is necessary not only to suppress a shift, the magnification, and rotational components of a shot array within the plane of a substrate, but also to suppress fluctuations in, e.g., magnification and distortion within shots. Fluctuations in, e.g., magnification and distortion are thought to be accounted for by thermal deformation of an original (reticle) and an optical element which constitutes a projection optical system when the original and optical element absorb exposure light.
FIG. 1A is a graph illustrating the result of evaluation of deformation of an original as a magnification fluctuation attributed to the deformation. FIG. 1B is a graph illustrating the result of evaluation of deformation of an optical element in a projection optical system. In FIGS. 1A and 1B, the abscissa indicates the time for which the original and optical element receive exposure light. Upon receiving the exposure light, the original and optical element absorbs the exposure light and so heat up and deform. This deformation causes a magnification fluctuation. A magnification fluctuation βr attributed to deformation of the original reaches a saturation magnification βrs after the elapse of a time τr. A magnification fluctuation β1 attributed to deformation of the optical element in the projection optical system reaches a saturation magnification β1s after the elapse of a time τ1.
In the magnification fluctuations shown in FIGS. 1A and 1B, the time constants τr and τ1 each are a function of a thermometric conductivity α(=k/pc where k is the thermal conductivity, ρ is the density, and c is the specific heat) of a material, and therefore stay constant even when energies absorbed by the original and projection optical element increase in response to a change in exposure condition. In view of this, as long as these time constants are calculated in advance, the deformation amounts of the original and optical element can be predicted from the time constants and the thermal loads (the exposure energies, the irradiation times, and the non-irradiation times) on the original and optical element, which dynamically change upon exposing the substrate by an exposure apparatus.
Japanese Patent Laid-Open No. 10-199782 discloses a method of correcting the magnification of a projection system based on a change curve describing thermal deformation of an original (reticle) for the exposure time and that describing thermal deformation of the projection system for the exposure time. In this method, the time constants of the original and projection system are calculated in advance. During a substrate exposure process, the deformation amounts of the original and projection system can be predicted based on the change curves and the times for which the original and projection system are irradiated with exposure light. Change curves describing thermal deformation of the original and projection system can be obtained based on the exposure results (the amounts of change in exposure shot magnification) obtained by, for example, exposing the original and projection system by imposing thermal loads on them in a cool state.
The deformation amount of an original when it is subjected to a thermal load can be predicted in accordance with change curves as mentioned above. However, an actual original often does not have a shape conforming to its design even while a thermal load imposed on the original upon exposure is zero (even before exposure). This is accounted for by the temperature difference between the interior and exterior of a chamber in the exposure apparatus. An original has a temperature that has not been equal to the temperature in the chamber for an appropriate time after the original is loaded from the outside of the chamber to its inside. Hence, an original which has just been loaded from the outside of the chamber to its inside is deformed with reference to the shape of the original at the temperature in the chamber.
The temperature difference between the interior and exterior of the chamber may be, for example, about 2° C. to 3° C. The magnification fluctuations illustrated in FIGS. 1A and 1B bear no information of the deformation amount of the original attributed to the temperature difference between the interior and exterior of the chamber (the deformation amount with reference to the shape of the original at the temperature in the chamber).
For example, as illustrated in FIG. 2, assume that the magnification error attributed to deformation of the original is β0 before exposure due to the temperature difference between the interior and exterior of the chamber in the exposure apparatus. A conventional method does not take account of magnification error that has been generated before exposure due to the temperature difference between the interior and exterior of the chamber. Accordingly, as indicated by a solid line 200, the result of addition of a magnification fluctuation corresponding to the change curve illustrated in FIG. 1A to β0 is determined as an actual magnification fluctuation by mistake, and the magnification of the projection optical system is controlled so as to correct the calculated magnification fluctuation. In reality, a magnification fluctuation asymptotically converges to βrs.